The present invention relates to a photosemiconductor device and a method for fabricating the photosemiconductor device, more specifically to a photosemiconductor device having quantum dots and a method for fabricating the photosemiconductor device.
Semiconductor light amplifiers and semiconductor lasers are small-sized and have small electric power consumptions, which makes them attractive in the fields of optical communication, etc.
A conventional semiconductor light amplifier will be explained with reference to FIG. 9. FIG. 9 is a sectional view of the conventional semiconductor light amplifier.
As shown in FIG. 9, a clad layer 112 of n type InP is formed on a semiconductor substrate 110 of n type InP. A bulk active layer 124 of InGaAs is formed on the clad layer 112. A clad layer 136 is formed on the bulk active layer 124. The clad layer 136, the bulk active layer 124 and the clad layer 112 are formed in a mesa as a whole and form a mesa-shaped light waveguide layer 138. A current constriction layer 118 is formed of a p type InP layer 118a and an n type InP layer 118b on both sides of the light waveguide layer 138. A cap layer 140 is formed of n type InP on the light waveguide layer 138 and the current constriction layer 118. AR (Anti-Reflection) coat film (not shown) is formed on both sides of the mesa-shaped light waveguide layer 138. The conventional semiconductor light amplifier has such structure.
The use of a quantum well active layer in place of the bulk active layer 124 is also proposed. The use of the quantum well active layer can improve gains in comparison with the use of the bulk active layer.
However, the gain bandwidth of the conventional semiconductor light amplifier using the bulk active layer and the quantum well active layer is small. Accordingly, the conventional semiconductor light amplifier cannot amplify a WDM (Wavelength Division Multiplexing) signal of a wide band at once.
Here, injecting more current into the bulk active layer and the quantum well active layer increases numbers of electrons and holes stored in the bulk active layer and the quantum active layer, which will widen the gain bandwidth. However, the increase of the injected current to the bulk active layer and the quantum well active layer increases calorific powers, and temperatures of the bulk active layer and the quantum well active layer rise. Thus, there is a limit to widening the gain bandwidth by injecting more current into the bulk active layer and the quantum well active layer.
Decreasing a thickness of the bulk active layer or decreasing a layer number of the quantum well active layer decreases a state density of carriers per a unit area of the current injection region, whereby a Fermi level could be transferred to a higher energy side, and the gain bandwidth could be widened. However, even decreasing, e.g., a layer number of the quantum well active layer to one layer will widen the gain bandwidth to about 70 nm at maximum.
Here, a high reflection film (not shown) is formed on both side surfaces of the mesa-shaped light waveguide layer 138 to thereby form a semiconductor laser. However, the conventional semiconductor lasers using the balk active layer and the quantum well active have narrow gain bandwidths and accordingly narrow wavelength variable ranges.
An object of the present invention is to provide a photosemiconductor device of a wide gain bandwidth, and a method for fabricating the photosmeiconductor device.
According to one aspect of the present invention, there is provided a photosemiconductor device comprising a plurality of quantum dots, the plurality of quantum dots having disuniform sizes.
According to another aspect of the present invention, there is provided a method for fabricating a photosemiconductor device comprising the step of: forming a plurality of quantum dots of disuniform sizes on a semiconductor substrate.
As described above, according to the present invention, the quantum dots of disuniform sizes are formed by a low area ratio, whereby the photosemiconductor device can have a wide gain bandwidth.